An integrated air ground simulation with commercial airline pilots and certified professional controllers was conducted at NASA Ames Research Center to evaluate a concept for air-ground trajectory negotiation. This concept was developed as part of the Distributed Air-Ground Trajectory Negotiation Project, which explores use of new technology, including CPDLC and flight deck and ATC decision support tools, to accommodate user preferred trajectories. Two human-in-the-loop simulation studies were conducted in 2002 and 2003. The first study in 2002 focused on how an integration of air and ground side decision support tools (DSTs) with data link can potentially improve efficiency, capacity, and workload distribution. The second study in 2003 focused on pilot/controller interactions during a trajectory negotiation. The results from the 2002 study suggested that this concept allowed for more precise delivery, efficient flight paths, and lower controller workload, while the 2003 study demonstrated the feasibility of trajectory negotiation via data link. This paper summarizes these results, discusses critical factors that contribute to the success of the concept, and open issues that need to be understood in order to further the concept. Overall, integration of DSTs and data link seems to show great potential. The trajectory negotiation concept appears feasible but its potential for benefits needs further research.